The present invention relates generally to hoisting equipment of the type used on oil and gas derricks for raising and lowering pipe, casing, and tubing, and specifically to an improved slip-type elevator and locking mechanism therefor.
In a typical derrick arrangement, a traveling block is suspended from the derrick crown block by a series of cables which are driven by the derrick drawworks to raise and lower the traveling block along the vertical axis of the derrick. The usual derrick hook is suspended from the traveling block and supports a derrick elevator by means of links. The derrick elevator has a flat upper surface for supporting pipes to be raised or lowered, typically at an upset area of the pipe exterior such as a tool joint. In certain of the elevator designs, particularly the so-called elevator-spider, the elevator has a tapered interior "bowl" and a series of gripping dies or "slips" which are pivotally moved up and down within the bowl to grip the exterior surface of a pipe being handled.
In a typical operation in which casing is being run into a well bore, a pair of elevator-spiders are used in tandem. The lower elevator-spider rests on the derrick floor and supports the casing string in the well bore by means of its slips which are set to grip the casing exterior. A new joint of casing is raised into position over the well bore by means of an auxiliary elevator and the lower end of the casing joint is connected to the upper end of the casing string in the well bore. The upper elevator-spider is then stripped down over the top of the casing joint and the slips are set to grip the casing exterior. The upper elevator spider is then used to lift up the casing string which releases the slips of the lower elevator-spider and the casing string is lowered into the well bore. The slips of the lower elevator-spider are then set to support the casing string in the well bore and the upper elevator-spider is disengaged and stripped up and off the casing to allow another casing joint to be moved into position. This cycle is repeated until all the casing is run into the well bore.
Slip-type elevators generally have slips which are pivotally operable between a "slips-up" position and a "slips-down" or set position for gripping the pipe exterior. The slips are moved between the up and down positions by means of a fluid cylinder arrangement. In a typical arrangement, a yoke is connected to the slips by suitable linkages. The output shafts of a pair of fluid cylinders are connected on one side of the yoke with the opposite side of the yoke being connected to the slip linkages. The yoke pivots about a pivotal axis in the approximate center of the yoke when fluid pressure is applied to the cylinders. During normal raising or lowering of the slips, the fluid cylinders serve to retain the slips in the raised or lowered position. The rate of raising or lowering of the slips can be controlled by providing an adjustable air flow valve or valves in the air distribution system leading to the fluid cylinders.
Because of the great weight of the pipe, casing, and tubing which is raised and lowered on the derrick and the danger to rig personnel, there exists a need for a locking mechanism which will lock the slips in the event of a loss of fluid pressure. Prior locking mechanisms were manually operated by rig personnel. Because the mechanisms did not operate automatically upon loss of fluid pressure, operator dilligence was required to insure that the slips did not open or set prematurely. Elimination of this human factor would increase safety and reliability of the elevator operation.